Return-Path: linux-nfs-owner@vger.kernel.org Received: from fieldses.org ([174.143.236.118]:34340 "EHLO fieldses.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932796Ab2F2U3L (ORCPT ); Fri, 29 Jun 2012 16:29:11 -0400 Date: Fri, 29 Jun 2012 16:29:03 -0400 From: "J. Bruce Fields" To: NeilBrown Cc: Al Viro , Nick Piggin , Nick Piggin , linux-nfs@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [PATCH] fs/dcache: allow __d_obtain_alias() to return unhashed dentries Message-ID: <20120629202903.GB17103@fieldses.org> References: <20101227234641.GA22248@fieldses.org> <20110118204509.GA10903@fieldses.org> <20110118220817.GF10903@fieldses.org> <20110308181320.GA15566@fieldses.org> <20110310105821.GE22723@ZenIV.linux.org.uk> <20110311150749.2fa2be66@notabene.brown> <20120628135927.GA6406@fieldses.org> <20120629201034.GA17103@fieldses.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20120629201034.GA17103@fieldses.org> Sender: linux-nfs-owner@vger.kernel.org List-ID: On Fri, Jun 29, 2012 at 04:10:34PM -0400, J. Bruce Fields wrote: > On Thu, Jun 28, 2012 at 09:59:27AM -0400, J. Bruce Fields wrote: > > Coming back to this now, just trying to review the > > filehandle-lookup/dcache interactions: > > > > On Fri, Mar 11, 2011 at 03:07:49PM +1100, NeilBrown wrote: > > > 1/ Originally DCACHE_DISCONNECTED didn't really mean much - it's presence > > > was only a hint, its absence was a strong statement. > > > If the flag is set, the dentry might not be linked to the root. > > > If it is clear, it definitely is link through to the root. > > > However I think it was used with stronger intent than that. > > > > > > Now it seems to mean a little bit more: If it is set and the dentry > > > is hashed, then it must be on the sb->s_anon list. > > > > The code that makes that assumption is __d_shrink (which does the work > > of d_drop)--it uses DCACHE_DISCONECTED to decide which hash chain to > > lock. > > > > I can't find any basis for that assumption. The only code that clears > > DCACHE_DISCONNECTED is in expfs.c, and it isn't done at the same time as > > hashing. Am I missing something? > > > > > This is a significant > > > which I never noticed (I haven't been watching). Originally a > > > disconnected dentry would be attached (and hashed) to its parent. Then > > > that parent would get its own parent and so on until it was attached all > > > the way to the root. Only then would be start clearing > > > DCACHE_DISCONNECTED. It seems we must clear it sooner now... I wonder if > > > that is correct. > > > > It looks wrong to me: > > > > If we clear DCACHE_DISCONNECTED too early, then we risk a filehandle > > lookup thinking the dentry is OK to use. That could mean for example > > trying to rename across directories that don't have any ancestor > > relationship to each other in the dcache yet. > > > > So we need to wait to clear DCACHE_DISCONNECTED until we *know* the > > dentry's parents go all the way back to the root. As you say, that's > > what the current code does. > > > > But that means DCACHE_DISCONNECTED dentries can be hashed to their > > parents, and __d_shrink can be handed such dentries and then get the > > locking wrong. > > > > It looks like this bug might originate with Nick Piggin's ceb5bdc2d246 > > "fs: dcache per-bucket dcache hash locking"? There's no discussion in > > the changelog, so probably it was just based on an unexamined assumption > > about DCACHE_DISCONNECTED. > > > > I wonder if an IS_ROOT() test could replace the DCACHE_DISCONNECTED test > > in __d_shrink(), or if we need another flag, or ? > > Bah, sorry, and I only just noticed that you already said as much later > and did the IS_ROOT() thing in your patch. > > Anyway, here's just that one change with a slightly more painstaking > changelog. > > --b. > > commit b1fa644355122627424fe2240a9fc60cbef4c349 > Author: J. Bruce Fields > Date: Thu Jun 28 12:10:55 2012 -0400 > > dcache: use IS_ROOT to decide where dentry is hashed > > Every hashed dentry is either hashed in the dentry_hashtable, or a > superblock's s_anon list. > > __d_shrink assumes it can determine which is the case by checking > DCACHE_DISCONNECTED; this is not true. > > It is true that when DCACHE_DISCONNECTED is cleared, the dentry is not > only hashed on dentry_hashtable, but is fully connected to its parents > back to the root. > > But the converse is *not* true: fs/exportfs/expfs.c:reconnect_path() > attempts to connect a directory (found by filehandle lookup) back to > root by ascending to parents and performing lookups one at a time. It > does not clear DCACHE_DISCONNECTED until its done, and that is not at > all an atomic process. > > In particular, it is possible for DCACHE_DISCONNECTED to be set on a > dentry which is hashed on the dentry_hashtable. > > Instead, use IS_ROOT() to check which hash chain a dentry is on. This > *does* work: > > Dentries are hashed only by: > > - d_obtain_alias, which adds an IS_ROOT() dentry to sb_anon. > > - __d_rehash, called by _d_rehash: hashes to the dentry's > parent, and all callers of _d_rehash appear to have d_parent > set to a "real" parent. > - __d_rehash, called by __d_move: rehashes the moved dentry to > hash chain determined by target, and assigns target's d_parent > to its d_parent, before dropping the dentry's d_lock. > > Therefore I believe it's safe for a holder of a dentry's d_lock to > assume that it is hashed on sb_anon if and only if IS_ROOT(dentry) is > true. > > I believe the incorrect assumption about DCACHE_DISCONNECTED was > originally introduced by ceb5bdc2d246 "fs: dcache per-bucket dcache hash > locking". > > Cc: Neil Brown > Cc: Nick Piggin > Signed-off-by: J. Bruce Fields > > diff --git a/fs/dcache.c b/fs/dcache.c > index 87c2da7..b2b382c 100644 > --- a/fs/dcache.c > +++ b/fs/dcache.c > @@ -410,7 +410,7 @@ static void __d_shrink(struct dentry *dentry) > { > if (!d_unhashed(dentry)) { > struct hlist_bl_head *b; > - if (unlikely(dentry->d_flags & DCACHE_DISCONNECTED)) > + if (unlikely(IS_ROOT(dentry->d_flags))) Um, right--I'll send an actual tested version along with some other stuff later. --b. > b = &dentry->d_sb->s_anon; > else > b = d_hash(dentry->d_parent, dentry->d_name.hash);